Prior milling cutters for cutting a T shaped groove are provided with two cutting edges in case the diameter of the hole to be cut is relatively small and are provided with four cutting edges in case the diameter of the hole to be cut is relatively large and the feed speed is increasd to reduce the load on each cutting edge, the material of the cutting edge being cemented carbide which is not easily broken. On the other hand, in order to eliminate any place being not cut, the cutting range of the two cutting edges 3,4 is overlapped in the length of C.sub.2 and cutting portion 31 of cutting edge 3 and cutting portion 41 of cutting edge 4 are regulated to be aligned (in a line in FIG. 8). When four cutting edges are employed, there are two cutting edges 3 and two cutting edges 4, the two pairs of cutting edges being able to cut the same place respectively.
In the prior art, as shown in FIG. 8 which is a schematic elevation observed by a stationary one at a certain part of the T shaped groove cutter holding portion 1a from the front side and the milling cutter is rotated by a certain angle to the same position and the two views are overlapped, and as explained before the cutting edges 3,4 cut with an overlapped length C.sub.2. Besides, cutting portion 31 (C.sub.2 plus C.sub.3) of cutting edge 3 and cutting portion 41 (C.sub.2 plus C.sub.4) of cutting edge 4 form a difference B.sub.2 in level as shown in FIG. 5 which is a schematic elevation observed by a stationary one at a certain part of the T shaped groove cutter holding portion 1a from the front side and the milling cutter is rotated by a certain angle to the same position and the two views are overlapped, one of the cutting edges (cutting edge 4 in FIG. 5) touches by its total length (cutting portion 41 i.e. C.sub.2 plus C.sub.4) causing the cutting edge (cutting edge 4 in FIG. 5) a large amount of load and violent damage. Therefore, in fitting the milling cutter for T shaped groove and cutting edges, in order to eliminate the difference B.sub.2 in level of FIG. 5 which is a schematic elevation observed by a stationary one at a certain part of the T shaped groove cutter holding portion 1a from the front side and the milling cutter is rotated by a certain angle to the same position and the two views are overlapped, i.e. in order to put the cutting portions 31 and 41 in a line, severe accuracy is required.
But, the length C.sub.2 is overlapped as shown in FIG. 8 and when the cutter is fed to the direction D, the length C.sub.3 of work is cut by cutting edge 3, the length C.sub.4 is cut by cutting edge 4 and the length C.sub.2 is cut by both of the cutting edges 3 and 4 half-and-half. Since a cutting amount difference is caused between the length C.sub.2 and C.sub.3 in the cutting edge 3 and between the length C.sub.2 and C.sub.4 in the cutting edge 4, the partial force received on the length of the cutting edge becomes non-uniform. Thereby an increase of feed speed is difficult, and the non-uniformity of force causes increased damage and wear of the cutting edge and as it is difficult to predict when the damage occurs, the reliability of the cutting edge is low and its stock control is also difficult.
Up to now, cutting edges made of high rigidity cemented carbide have been used, but the cost per one cutting edge is quite high and the violent damage and wear have prevented the economical usage of expensive cutting edges.
On the other hand, since it is difficult to adjust the cutting edge mounting seat after the hardening of the milling cutter (there is no way to adjust), the milling cutter for cutting T shaped groove is not hardened sufficiently and the cutting portions 31 and 41 are adjusted so as to be aligned in a line as shown in FIG. 8. Therefore the hardness of the milling cutter itself becomes low and there is a fear of deformation.